Possible GMB Bug Prevents Users Being Added As Managers On Location Groups
https://support.google.comLooks like there might be an issue with users not being able to add location groups as managers to listings in Google My Business.
Google has not confirmed if this is a bug. Also, this is not to be confused with the site manager role that was recently deprecated.
Some users have noticed that you can get added as an owner and then downgrade to manager.